Metals including tungsten have diverse applicability in manufacturing integrated circuits. For example, metal layers can be formed to interconnect numerous conductive devices positioned on a siliconwafer or to constitute them. Currently, integrated circuits are produced by mounting appropriate devices such as a diode and a transistor on and/or in a siliconwafer. At this time, processes for forming separate layers of insulating material and conductive material throughout the surface of the device are repeated. In an integrated circuit, characteristics of a contact hole or via depend on the nature of the insulating material, and the via is filled with conductive materials so as to provide a vertical contact that penetrates the insulating material and contacts appropriate parts of the devices mounted on the wafer. While aluminum is useful as a wiring material, it is inadequate to fill the inside of a via completely. Typically, a via is filled with tungsten that can be adapted to a CVD (chemical vapor deposition) process. In the course of chemical vapor deposition, tungsten not only fills the via also covers the surface of the insulating layer. Following the elimination of the excess of tungsten, aluminum wiring is then formed on the dielectric layer and the via.
Previously, a RIE (reactive ion etching) process was employed in the tungsten-elimination step in conventional semiconductor processes. However, the RIE process has critical shortcomings in that it is apt to overetch the tungsten layer to partially etch even the tungsten in the via, resulting in poor contact with the aluminum wiring formed on the tungsten layer. Furthermore, any tungsten particles remaining on the wafer can cause fatal inferiority in the resulting semiconductor integrated circuit. It is the CMP(chemical mechanical polishing) process that has been developed to solve these problems with the RIE process.
The CMP process has come to be introduced in the art according to the increase of integrity and the number of layers of the semiconductor integrated circuits. In the CMP process, a semiconductor wafer is subjected to orbital movement combining rotation and translation while contacted with a polishing pad made of polyurethane and with a slurry composition comprising abrasives and various other compounds, to planarize the surface of the wafer. Generally, slurry compositions used in such CMP process (hereinafter, referred to as ‘CMP slurry’) provides both chemical and mechanical polishing effects, while etchants, oxidants, acids, and dispersing agents being in charge of the chemical polishing function and abrasives, micro-particles of metal oxides being in charge of the mechanical polishing function. These two polishing functions allow any protrusion of the outer surface of the wafer to be selectively etched and polished, resulting in the best planarization of the wafer.
According to the polishing substrate, CMP slurry can be classified into oxide-polishing slurry for insulating layers such as SiO2 layer and metal-polishing slurry for metal layers such as tungsten or aluminum layer. These two classes of CMP slurry are discriminated in the components exerting the chemical polishing function.
Meanwhile, the conventional metal-polishing CMP slurry has the typical problems represented by the oxide erosion due to the difference of polishing rate among a metal, a barrier metal and a dielectric layer, and the corrosion due to the excessive etch of metal layers by oxidant ingredients. If the erosion or the corrosion occurs during CMP process, contact with another metal layer deposited on the polished metal layer comes to be poor, so that the resulting semiconductor integrated circuit may not work normally. These problems are considered to arise mainly from the excessive oxidizing power of the oxidants contained in the polishing slurry. Up to date, polishing rate required for the conventional semiconductor processes has been accomplished primarily by increasing either the amount or the strength of oxidants added to a CMP slurry. Consequently, oxide erosion, corrosion, pit, dishing and so on were inevitably caused. Such oxide erosion and corrosion are schematically illustrated in FIGS. 1a and 1b, respectively.
Moreover, a metal CMP slurry is prepared in an acidic condition for the purpose of vigorous polishing of tungsten, and at such low pH usually decreases the dispersability of abrasive particles, which results in significant deterioration of the reproducibility of polishing performance of a slurry.